2,5-Furandicarboxylic acid (FDCA) is a diacid that can be produced from natural sources such as carbohydrates. Routes for its preparation using air oxidation of 2,5-disubstituted furans such as 5-hydroxymethylfurfural or ethers thereof with catalysts comprising Co and Mn have been disclosed in e.g. WO2010/132740, WO2011/043660 and WO2011/043661.
U.S. Pat. No. 2,551,731 describes the preparation of polyesters and polyester-amides by reacting glycols with dicarboxylic acids of which at least one contains a heterocyclic ring, such as 2,5-FDCA. Under melt polymerization conditions, using sodium- and magnesium methoxide as a catalyst, FDCA and 2.5 equivalents of ethylene glycol or FDCA dimethyl ester and 1.6 equivalents of ethylene glycol were reacted in a esterification step or transesterification step, respectively, at ambient pressure between 160 and 220° C., after which a polycondensation was carried out between 190 and 220° C. under a few mm Hg pressure. The polycondensation process took between about 5 to over 7 hours. The product had a reported melting point of 205-210° C. and readily yielded filaments from the melt.
In US 2009/0124763 polyesters are described, having a 2,5-furandicarboxylate moiety within the polymer backbone and having a degree of polymerization of 185 or more and 600 or less. These polymers are made in a three step process involving the esterification of the 2,5-FDCA or the transesterification of the diester thereof with a diol, and a second step involving polycondensation, followed by solid state polymerization as third step.
The first step is carried out at ambient pressure at a temperature within a range of 150 to 180° C., whereas the polycondensation step is carried out under vacuum at a temperature within a range of 180 to 230° C. The product is then purified by dissolving the same in hexafluoroisopropanol, re-precipitation and drying, followed by the third step, a solid state polymerization at a temperature in the range of from 140 to 180° C. For the preparation of poly(ethylene furandicarboxylate) the first two steps took over 11 hours.
In WO 2010/077133 a process for preparing furandicarboxylate-containing polyesters is described wherein the diester of FDCA is transesterified with a diol, and the ester composition thus obtained is subjected to polycondensation. The polycondensation is conducted for a period of up to 5 hours. The polycondensate may then be subjected to solid state polymerization. In an example the solid state polymerization was conducted for 60 hours. Although the molecular weight of the polyester obtained is reasonably high, the duration of the solid state polymerization is considered too long. An improvement is described in WO 2013/062408, wherein the dimethyl ester of FDCA is transesterified with ethylene glycol, or bis(2-hydroxyethyl)-2,5-furandicarboxylate is used as starting material. The transesterification product or this starting material is then subjected to polycondensation and after a drying/crystallization step the polycondensate is subjected to solid state polymerization. The polycondensation was shown to take three hours. In an example the solid state polymerization takes two days.
In WO 2013/120989 a continuous process for the preparation of poly(ethylene furandicarboxylate) is described wherein FDCA or a diester thereof is mixed with ethylene glycol at elevated temperature to give a paste or a homogeneous solution, the paste or solution is converted to an esterification product of FDCA and ethylene glycol, the esterification product is polycondensed under reduced pressure, wherein the polycondensation is performed in two stages. According to an example the dimethyl ester of FDCA was reacted with ethylene glycol in a molar ratio of 1:1.7. In this example the stages following the production of the esterified product took 5 hours. The polycondensation product can be subjected, if desired, to a solid stating polymerization.
KR 20140003167 describes a polyester polymer with excellent transparency which is manufactured by using a biomass originated furandicarboxylate ester compound with ethylene glycol. In comparative examples also furandicarboxylic acid has been used. The molar ratio of furandicarboxylate ester to ethylene glycol may be from 1:1.1 to 1:4. The ratio of furandicarboxylic acid to ethylene glycol varies between 1:1.2 to 1:2. No indication is provided that specific measures have been taken to reduce the content of diethylene glycol in the resulting polyester.
In U.S. Pat. No. 8,420,769 polyesters are presented that have been prepared from FDCA or the diester thereof with a mixture of ethylene glycol and diethylene glycol. The amount of diethylene glycol is at least 50.1% mol with respect to the combination of ethylene glycol and diethylene glycol. The preparation process may take as long as 8.5 hours. The resulting polyester is stated to have improved impact strength. In a comparative experiment is has been shown that when no diethylene glycol is added as comonomer, the resulting polyester still shows small peaks in the 1H-NMR spectrum at shifts of about 4.2 and 4.8 ppm, indicating diethylene glycol moieties. From the peaks it can be deduced that the amount of diethylene glycol moieties is about 0.05 mol/mol, based on the amount of furandicarboxylate moieties.
This patent document confirms the finding by the Applicants that during the formation of the esterification product of FDCA and ethylene glycol, diethylene glycol is readily formed, which is subsequently built into the polyester that is obtained during the following polycondensation step and optional solid stating step.
Applicants have found that the incorporation of diethylene glycol moieties in the polyester reduces the melting point, reduces the glass transition temperature and crystallization level. Since the crystallization level is known to have an effect on the mechanical properties of the articles formed from such polyesters, it is believed that the incorporation of diethylene glycol moieties into the polyesters reduce the thermal stability and mechanical properties of such articles. When a polyester with a reduced content of diethylene glycol moieties is produced it has been found that the negative effects on the melting point, thermal stability and mechanical properties is reduced. Hence, contrary to what is being taught by U.S. Pat. No. 8,420,769 a thermally more stable polyester having improved mechanical properties can be produced by reducing the amount of diethylene glycol moieties instead of increasing this amount.